Surgeons typically "vent" or drain the left ventricle of the heart during cardiopulmonary bypass operations in order to reduce the pressure in the left ventricle, which otherwise may lead to overdistention (excessive stretching) of the left side of the heart and pulmonary veins. For example, the left ventricle may be vented with a catheter that is connected to an extracorporeal support system, including a pump for draining the chamber.
Vent catheters have conventionally been introduced into the left ventricle with the assistance of a relatively stiff stylet extending through the lumen of the catheter distally or outwardly beyond the distal end of the catheter. Such stylets provide sufficient rigidity to the catheter so that the catheter may be pushed through an incision into a pulmonary vein or directly through the wall of the heart, and the distal end of the stylet helps to stretch and open an opening through the vein or heart wall through which the catheter may be advanced. The catheter would then be positioned in the left ventricle with the distal end of the stylet extending outwardly beyond the distal end of the catheter.
One problem with such a procedure has been the risk of damaging heart tissue by scraping the tissue with the distal end or tip of the stylet, which is typically formed of relatively hard material, such as nylon. At least one surgeon has suggested performing cardiopulmonary bypass operations without venting the left ventricle to avoid this problem, notwithstanding the fact that left heart pressures are substantially higher without venting. See, for example, H. Zwart, J. Brainard, and R. DeWall, Ventricular Fibrillation Without Left Ventricle Venting, 20 The Annals of Thoracic Surgery 418-423 (October 1975).
One approach to this problem is shown in U.S. Pat. No. 4,834,709, which describes a soft silicone catheter having a closed rounded distal end that may be introduced into the left ventricle of the heart without the distal end of a stylet being exposed to heart tissue. Such a catheter protects heart tissue by effectively shielding the stylet within the relatively soft catheter. However as a result of closing the distal end of the catheter, the potential flow rate of fluid through the catheter is reduced relative to catheters that have open distal ends, and the stylet is rendered incapable of opening or stretching an opening through a vein or the heart wall.
Another approach has been to retract the stylet from its fully inserted position, where its distal end extends beyond the catheter, into the catheter so that the catheter houses or shields the distal end of the stylet. The catheter would then be inserted through an incision into a vein, artery or wall of the heart, without the distal end of the stylet stretching the incision. As a result, it is more difficult to insert the catheter through the vein, artery or heart wall than would be the case when the distal end of the stylet extends beyond the distal end of the catheter.
Relatively stiff vent catheters have also been used without stylets during cardiopulmonary bypass operations. Stiffness may be provided by embedding a malleable metal wire in the wall of the catheter. An urethral catheter having such an embedded metal wire is described in U.S. Pat. No. 3,867,945. Sarns, Inc., a subsidiary of Minnesota Mining and Manufacturing Company of St. Paul, Minn., has been selling a left heart vent catheter that has a preformed curved section generally adjacent the distal end of the catheter. This preformed catheter has sufficient rigidity for introduction into the heart without a stylet. However, soft, very flexible catheters are frequently desired for use in cardiopulmonary bypass operations, since the heart can be more readily manipulated and repositioned without interference from the catheter.